Polymer blend membranes with improved mechanical properties

ABSTRACT

Improved polymer blends, and polymer blend membranes having improved mechanical properties are provided by alteration of the critical solution temperature of the polymer blend spinning solution through the incorporation of organic or inorganic complexing agents.

This application is a divisional of application Ser. No. 09/306,824,filed on May 7, 1999, which is a divisional of application Ser. No.08/994,041, filed on Dec. 19, 1997 (now U.S. Pat. No. 5,922,791).

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to polymer blend, hollow fiber membranes havingimproved mechanical properties by alteration of the thermodynamic phaseequilibrium of the concentrated polymer blend spinning solution throughthe incorporation of organic or inorganic complexing agents.

2. Description of the Related Art

The science of polymer blend miscibility and the prediction of the typesof phase diagrams for polymer blends in solutions are quite complex.Unusual two-peaked coexistence curves and phase diagrams with a tendencytoward greater miscibility at intermediate temperatures are reported inthe literature for different polymer blends. The following articles andtextbooks depict the complexity of the theory of polymer blendmiscibility and provide several examples of different thermodynamicphase diagrams for polymer blends:

R. L. Scott, J. Chem. Physics, Vol. 17, p. 279 (1949);

D. R. Paul & S. Newman, Polymer Blends, Vols. 1 and 2, Academic Press,San Francisco, London (1978);

0. Olabasi et al., Polymer—Polymer Miscibility, Academic Press, NY(1979); and

Pierre-Gillesde Gennes, Scaling Concents in Polymer Physics, Chapter 4,Sections 4.1 and 4.2, Cornell University Press (1979).

Polymer blends which are molecularly compatible in the solid state attemperatures below the glass transition temperature of the blend mightexhibit a thermodynamic phase equilibrium consisting of a coexistencecurve in the solution state. The coexistence curve is the locus of thecritical solution temperature (CST) as a function of the blendcomposition.

A particular blend solution might exhibit a coexistence curve for thehigher critical solution temperature (HCST), or for the lower criticalsolution temperature (LCST), or for both the HCST and the LCST. Forany-specific blend composition, the blend solution is two-phase if thetemperature is below the HCST or if the temperature is above the LCST(assuming that the phase diagram contains coexistence curves both forthe HCST and LCST). For that type of phase diagram, the blend solutionis single phase for any specific blend composition if the blend solutiontemperature is above the HCST and below the LCST.

U.S. Pat. No. 5,047,487 issued to Camargo et al. discloses that ULTEM1000, a polyetherimide available from GE, and MATRIMID 5218, aphenylindane-containing polyimide available from Ciba, are molecularlycompatible in the solid state. The molecular scale compatibility of thetwo polymers over the entire blend composition range was characterizedby Camargo et al. utilizing the technique of Differential ScanningCalorimetry (DSC). The ULTEM/MATRIMID blends exhibit a single glasstransition temperature (T_(g)) located in between the T_(g) of theindividual blend components over the entire blend composition range,which indicates molecular-scale blend miscibility.

U.S. Pat. No. 5,085,676 issued to Ekiner et al. discloses solutionspinning of hollow fiber membranes from concentrated ULTEM/MATRIMIDblend solutions. U.S. Pat. No. 5,443,728 discloses membranes preparedfrom blends of polyetherimide and phenylindane-containing polyimides.

In prior practices, blend solutions have been stored above the HCST toensure phase homogeneity. This may unfortunately result in polymerdegradation reactions due to sensitivity of the blend polymers to hightemperature exposure for prolonged times. The degradation of the blendpolymer adversely affects the final product properties.

On the other hand, storage at temperatures below the HCST results in atwo-phase solution. With prior methods, additional processing steps forheating and mixing of the two-phase blend solution were needed forformation of a homogenous blend solution prior to the final processingstep. A two-phase blend polymer phase morphology in the solution statewould also adversely affect the fiber spinning process continuity andthe final product properties.

ULTEM and MATRIMID polyimide solutions are particularly susceptible tomolecular weight degradation reactions in the solution state whenexposed to elevated temperatures for prolonged times.

It is, therefore, an object of the present invention to provide apolymer blend that does not suffer from the disadvantages mentionedabove. In particular, it is an object of the present invention todepress the HCST of a polymer blend solution in order to enhance itsphase stability during low temperature storage prior to processing. Itis a further object of the present invention to depress the HCST of theULTEM/MATRIMID blend solution formulations in order to permit storage ofthe solutions at lower temperatures without phase separation prior tosolution spinning into hollow fiber form.

These and other objects of the invention will become apparent in lightof the following specification, the figures, and the claims appendedhereto.

SUMMARY OF THE INVENTION

In one of its composition aspects, the present invention relates to apolymer blend comprising a plurality of polymers and a CST adjustmentagent. As used herein, a CST adjustment agent is an organic or inorganiccomplexing agent which enhances the miscibility of two or more polymersin solution.

Polymer blends particularly suited for use in the present inventioninclude polyimide blends such as a blend of polyetherimide andphenylindane-containing polyimide. In a preferred embodiment, thepolymer blend comprises two commercially available polymers, ULTEM andMITRIMID. Preferably, the polymer blend comprises between about 80% and95% by weight of polyetherimide and the balance phenylindane-containingpolyimide.

The CST adjustment agent is preferably an alkali or alkaline earth metalhalide such as ZnCl₂, CaBr₂, and LiCl.

The CST adjustment agent can also be organic; in which case,triethylamine is particularly preferred.

It has been surprisingly found that through the use of a CST adjustmentagent according to the present invention, the HCST of a polymer blendsolution is lowered by at least 10° C. relative to the same blendsolution without the addition of a CST adjustment agent.

In another of its composition aspects, the present invention relates toa polymer blend which has been annealed at elevated temperature. It hasbeen surprisingly discovered that annealing a polymer blend solution atelevated temperature has the effect of lowering the HCST of thesolution. The precise annealing time depends on the polymer blendsolution employed. Such times can be determined by routine optimizationby those skilled in the art for the particular polymer blend solution.The polymer blend solution is preferably annealed at an elevatedtemperature between 50° and 140° C., and more preferably between 700°and 100° C. In this embodiment, the polymer blend may or may not containa CST adjustment agent.

In one of its method aspects, the present invention relates to a methodfor modifying the CST of a polymer blend solution by combining thepolymer compounds of the polymer blend together with a solvent and a CSTadjustment agent. The resulting CST-modified polymer blend solution maybe used to form hollow fiber membranes having improved mechanicalproperties.

In another of its method aspects, the present invention relates to amethod for lowering the CST of a polymer blend solution by annealing thesolution at elevated temperature.

In yet another of its method aspects, the present invention relates to amethod for the separation of gases from a mixture, preferably air. Themethod includes the step of bringing the gas mixture into contact with acomposite hollow fiber membrane formed from a polymer solutioncontaining a CST adjustment agent in accordance with the presentinvention, at an elevated pressure to preferentially permeate at leastone component of the gas mixture to produce at least one gaseousproduct.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a phase diagram of a 31% by weight total polymer contentULTEM/MATRIMID blend solutions in NMP solvent.

FIG. 2 is a graph of the N₂ permeate flow versus the N₂ tube feedpressure of composite hollow fibers prepared in Comparative Example 1and Example 1.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will now be described with particular reference toULTEM/MATRIMID blends. However, it should be recognized that the presentinvention is applicable to other polymer blends including blends ofpolyimides.

Effect of Organic and Inorganic Additives on the HCST of the BlendSolutions

In a particularly preferred embodiment, the present invention employs anorganic or inorganic complexing agent to change the phase behavior,namely, the LCST or HCST, of a concentrated ULTEM/MATRIMID blendsolutions in N-methylpyrrolidone (NMP) solvent. The complexing agentcomplexes with the polymer chains and the solvent NMP.

It was surprisingly found that polymer chain conformational structureand chain mobility in the solution state may be altered through theformation of complexes. As illustrated in the examples below, somecomplexing agents increase the HCST resulting in impaired miscibility atlow temperatures, while others decrease the HCST resulting in enhancedmiscibility at low temperatures. The LCST may likewise be altered if theblend solution exhibits a coexistence curve for the LCST. With thepresent invention, complexing agents have been determined which areeffective in altering the thermodynamic phase equilibrium of polymerblend solutions, useful in fiber spinning.

Referring now to the figures, FIG. 1 is a phase diagram of a 31% byweight total polymer content ULTEM/MATRIMID blend solutions in NMPsolvent. The phase equilibrium data shown in FIG. 1 were obtained byvisual observation for each single phase blend composition solutionheated to a temperature above the HCST by measurement of the solutiontemperature at which the solution starts to become cloudy as thesolution temperature approaches the HCST and finally becomes two-phaseas the solution temperature decreases below the HCST.

The same data could presumably be obtained by heating a two-phasesolution and measuring the temperature at which the blend solutionbecomes a single phase and clear. The transformation from thesingle-phase region to the two-phase region was found to be very sharp,while the transformation from the two-phase region to the single-phaseregion was broad and covered a wide temperature range.

As seen in FIG. 1, the ULTEM/MATRIMID blend solutions exhibit acoexistence curve for the HCST. The blend solutions may also exhibit acoexistence curve for the LCST at temperatures higher than the datashown in FIG. 1.

The effect of different organic and inorganic additives on thethermodynamic phase behavior of the concentrated 95:5 and 90:10ULTEM:MATRIMID blend solutions in NMP solvent are summarized in Tables 1and 2 below. The data shown in these tables clearly indicate that someof the additives enhance thermodynamic miscibility and some impairmiscibility, as evidenced by the decrease or increase in the HCST ofthese solutions with respect to the control. ZnCl₂, CaBr₂, LiCl, andtriethylamine are preferred completing agents because they enhance themiscibility of the ULTEM:MATRIMID blend solutions, while LiNO₃, NaBr,and tetramethylenesulfone are not preferred because they appear toimpair the miscibility of the same solutions.

TABLE 1 EXAMPLES OF ADDITIVES WHICH AFFECT THE HCST OF THEULTEM:MATRIMID BLEND SOLUTIONS HCST Solution (° C.) Control* 33Control* + 0.3% ZnCl₂ <25 Control* + 0.3% triethylamine 27 Control* +0.3% triethylamine + 2.3% LiNO₃ 57 Control* + 0.3% triethylamine + 439.3% tetramethylenesulfone Control* + 0.3% CaBr₂ <25 Control* + 0.3% NaI25 Control* + 2.3% NaI 35 Control* + 2.3% CaBr₂ 30 Control* + 2.3%ammonium iodide 32 Control* + 2.3% ammonium bromide 33 Control* + 0.3%ZnCl₂ + 38 9.3% tetramethylenesulfone Control* + 2.3% NaBr 49 Control* +2.3% ZnBr₂ 29 Control* + 2.2% ZnBr₂ + 31 6.2% tetramethylenesulfone(*Control = 31% (95:5 ULTEM:MATRIMID) Blend Solution in NMP)

TABLE 2 EXAMPLES OF ADDITIVES WHICH AFFECT THE HCST OF THEULTEM:MATRIMID BLEND SOLUTIONS HCST Solution (° C.) Control* 40Control* + 0.3% LiCl 28 Control* + 0.3% ZnCl₂ 25 Control* + 0.3%triethylamine 35 Control* + 0.3% LiNO₃ 50 Control* + 0.3% MgCl₂ 37Control* + 0.3% ZnCl₂ + 2.3% NaI + 25 6.2% tetramethylenesulfone 34%(90:10 ULTEM:MATRIMID) BLEND + 31 2.6% CaBr₂ in NMP 32.5% (90:10ULTEM:MATRIMID) BLEND in NMP 43 32.5% (90:10 ULTEM:MATRIMID) BLEND +2.5% CaBr₂ + 41 6.5% tetramethylenesulfone in NMP *(Control = 31% (90:10ULTEM:MATRIMID) Blend Solution in NMP)

Effect of Annealing on the HCST of the Blend Solutions

In another particularly preferred embodiment, the present inventionemploys annealing to change the phase behavior, namely, the LCST orHCST, of a concentrated ULTEM/MATRIMID blend solutions inN-methylpyrrolidone (NMP) solvent. The blend solutions may or may notcontain a complexing agent.

It is demonstrated below that controlled annealing of the freshlyprepared ULTEM/MATRIMID blend solutions containing the preferredcomplexing agents such as CaBr₂ at a temperature above the HCST of theblend solution results in a further depression in the HCST of thesolution. The data shown in Table 3 depicts the depression in the HCSTof a concentrated 90:10 ULTEM:MATRIMID blend solutions containing CaBr₂as the complexing agent.

TABLE 3 EFFECT OF ANNEALING ON THE HCST OF BLEND SOLUTIONS Time AnnealedTime Annealed HCST Sample at 70° C. (hrs) at 90° C. (hrs) (° C.)Control * 0 0 41 Control * Annealed 3 0 39 Control * Annealed 6 0 39Control * Annealed 9 0 37 Control * Annealed 24 0 33 Control * Annealed0 3 34 Control * Annealed 0 6 32 Control * Annealed 0 17 27 *Control =32.5% (90:10 ULTEM:MATRIMID) Blend + 2.4% CaBr₂ + 6.5%Tetramethylenesulfone in NMP. Control exposed to 80° C. for 2 hours and90° C. for 2 hours during preparation.

The data in Table 3 indicate that annealing of the blend solutioncontaining the complexing agent CaBr₂ enhances the thermodynamic phasemiscibility of the blend solution.

It was also determined that annealing of the ULTEM:MATRIMID blendsolution in NMP which does not contain any additives also enhances thethermodynamic miscibility of the solution, but to a lesser extent thanthe counterpart solution containing the preferred complexing agents. Asan example, a 31% (90:10 ULTEM:MATRIMID) blend solution in NMP preparedat room temperature exhibited an HCST of about 40° C. After annealingthe same solution at 70° C. for about 15 hours, the HCST was reduced to35° C.

As another example on the effect of annealing the blend solution, asolution containing 31% (90:10 ULTEM:MATRIMID) blend +0.31% MgCl₂ in NMPwhich was prepared at room temperature exhibited an HCST of about 38° C.After annealing the blend solution at 70° C. for about 15 hours, theHCST was reduced to 27° C.

While not wishing to be bound by any particular theory, it is believedthat the presence of the preferred complexing agents in the solutionfacilitates the break up of some type of polymer chain associations andenhances the thermodynamic miscibility. Annealing the solutions alsoappears to alter the polymer chain conformational structure in thesolution state. Annealing of the blend solution in the presence of thepreferred complexing agents appears to accelerate the transformation ofbreaking up of the polymer chain associations in the liquid state. Thistransformation occurs in the absence of any appreciable polymermolecular weight breakdown.

In the absence of polymer molecular weight breakdown, this thermodynamictransformation appears to be reversible. Long time storage of theannealed solutions at a temperature below the HCST of the blend solutionresults in a gradual increase in the HCST eventually approaching theHCST of the unannealed solution. The time scale for thethermodynamically reversible transformation is on the order of severalweeks.

Reannealing of the blend solution which exhibits an increase in HCST dueto the reversible thermodynamic transformation will lower the HCST ofthe solution to its initially annealed state. This transformation isreversible so long as there is no appreciable polymer molecular weightdegradation during the annealing step. The transformation in thesolution state accompanied with polymer molecular weight degradationduring the annealing step is not reversible. The ULTEM:MATRIMID blendsolutions in our tests which were exposed to temperatures in excess of125° C. for more than 2 hours undergo polymer molecular weightdegradation, and exhibit a permanent reduction in HCST.

TABLE 4 EFFECT OF ANNEALING ON ULTEM AND MATRIMID MOLECULAR WEIGHT ANDMOLECULAR WEIGHT DISTRIBUTION Annealing Time Sample at 100° C. (hrs)M_(n) M_(w) M_(w)/M_(n) ULTEM¹ - CONTROL 0 25K 50K 2 ULTEM¹ - ANNEALED 225K 50K 2 ULTEM¹ - ANNEALED 4 25K 49K 2 ULTEM¹ - ANNEALED 8 24K 47K 2ULTEM¹ - ANNEALED 12 23K 45K 1.9 MATRIMID² - CONTROL 0 41K 80K 2MATRIMID² - ANNEALED 2 39K 79K 2.1 MATRIMID² - ANNEALED 4 39K 79K 2MATRIMID² - ANNEALED 8 36K 73K 2 MATRIMID² - ANNEALED 12 36K 70K 2 ¹31%ULTEM + 2.3% CaBr₂ in NMP ²31% MATRIMID + 2.3% CaBr₂ in NMP

The data in Table 4 indicate that annealing of the ULTEM and MATRIMIDsolutions containing the preferred complexing agent at 100° C. does notcause any appreciable polymer molecular weight degradation for the timeperiods specified. This is especially true for ULTEM which appears to bemore stable to thermal annealing than MATRIMID. Since a preferredembodiment of this invention is 90:10 ULTEM:MATRIMID blend solution,annealing the same solution would enhance the thermodynamic miscibilityof the blend at temperatures below 100° C. It thus appears that thethermodynamic transformation of the polymer chain conformationalstructure in the solution state due to annealing occurs in the absenceof any appreciable polymer molecular weight degradation.

Composite Fiber Soinning Examples with Two-Phase and Single-Phase CoreSolutions

The chemical structures of the ULTEM and the MATRIMID polyimides whichwill be utilized in the following examples are shown below.

COMPARATIVE EXAMPLE 1

A core solution containing 31% total weight of a polymer blendcomprising 90:10 weight of a polymer A, ULTEM 1000 (commerciallyavailable from GE) and a polymer B, MATRIMID 5218 (commerciallyavailable from Ciba), 2.3% weight of LiNO₃, 9.3% weight oftetramethylenesulfone, 1.6% weight of acetic anhydride, and 0.03% weightof acetic acid in N-methylpyrrolidone was prepared. During preparationand degassing steps, the temperature of the core solution was notallowed to exceed 40° C. The HCST of a sample of the same core solutionwas measured to be about 55° C. Since this core solution waspurposefully maintained below the HCST, the ULTEM:MATRIMID blend coresolution was in the two phase region of the thermodynamic phase diagram.

In accordance with U.S. Pat. No. 5,085,676, this core solution wascoextruded at a rate of 125 cm³/hour through a composite fiber spinnerethaving fiber channel dimensions of outer diameter equal to 559 microns(5.59×10⁻⁴ meters) and inner diameter equal to 254 microns (2.54×10⁻⁴meters) at 80° C.

A separating polymer solution containing 26% weight of MATRIMID 5218polyimide, 7.8% weight of tetramethylenesulfone, 1.3% weight of aceticanhydride, and 0.26% weight of acetic acid in N-methylpyrrolidone wascoextruded at a rate of 15 cm³/hr. A solution containing 90% weight ofN-methylpyrrolidone in H₂O was injected into the bore of the fiber at arate of 45 cm³/hr. The nascent filament traveled through an air-gaplength of 3 cm at room temperature into a water coagulant bathmaintained at 25° C. and was wound up at a rate of 90 meters/minute.

The water-wet fiber was washed with running water at 50° C. for about 12hours and dehydrated as disclosed in U.S. Pat. No. 4,080,744 and U.S.Pat. No. 4,120,098. This specifically involved the replacement of waterwith methanol followed by the replacement of methanol with hexane anddrying in a vacuum oven (2.67 kPa) at room temperature followed bydrying at 100° C. The fibers were potted in an epoxy resin at both endsinside a straight steel tubing to provide gas feed through the bore ofthe fibers.

The fibers were tested for mixed gas O₂/N₂ (21/79) with 100 psi borefeed pressure at 21° C. The fibers exhibited the following gasseparation properties while producing an inerts-enriched product streamcontaining 95% inerts: O₂ Permeance=93 GPU; O₂/N₂ Selectivity=1.2.

The fibers were treated to seal defects protruding through the denseouter gas separating layer as disclosed in U.S. Pat. No. 4,230,463. Thisinvolved contacting the outer surface of the fibers with an iso-octanesolution containing 2% weight Sylgard-184 silicone elastomer availablefrom Dow Corning Corporation for about 30 minutes at room temperature.The iso-octane solution was drained and the fibers were allowed to airdry. The fibers were tested for mixed gas O₂/N₂ (21/79) with 100 psibore feed pressure at 21° C.

The fibers exhibited the following gas separation properties whileproducing an inerts-enriched product stream containing 95% inerts: O₂Permeance=32 GPU; O₂/N₂ Selectivity=1.2. The above permeation dataindicate that the composite fibers spun from a two-phase core solutioncould not be post-treated to the full O₂/N₂ selectivity of the MATRIMIDseparating polymer which has an intrinsic O₂/N₂ selectivity of about 7.

While not wishing to be bound by any theory, we believe the two-phasecore solution impairs the structural integrity of the composite fiberand interferes with the formation of an integral MATRIMID sheathstructure separating skin layer. The lack of post-treatability of thefiber is due to the presence of large defects in the fiber separatingskin structure which could not be completely sealed even after treatmentwith a relatively high Sylgard content solution. The fact that thecomposite fibers suffered a large reduction in the magnitude of thepermeance as a result of the post-treatment depicts that the fibers werecoated with the Sylgard polymer.

EXAMPLE 1

A core solution containing 32.50% total weight of a polymer blendcomprising 90:10 weight ULTEM 1000 and MATRIMID 5218, 2.4% weight ofCaBr₂, and 6.5% weight of tetramethylenesulfone in N-methylpyrrolidonewas prepared. During the preparation of the solution, the solution wasexposed to 80° C. for about 2 hours and 90° C. for an additional 2hours. The core solution was then degassed and annealed at 70° C. forabout 12 hours. The HCST of this core solution was measured to be about37° C.

During the mixing and annealing steps, this core solution was kept abovethe HCST, and therefore, the core solution was in the single-phaseregion of the thermodynamic phase diagram. This core solution wascoextruded at a rate of 125 cm³/hour with the same separating polymersolution described in Comparative Example 1, which was coextruded at 15cm³/hr through the same composite fiber spinneret at 80° C. The samebore fluid composition and the spinning process conditions described inthe Comparative Example 1 were used, except that the air-gap lengththrough which the nascent fiber traveled was kept at 5 cm.

The fibers were washed, dehydrated and tested for mixed gas O₂/N₂(21/79) with 100 psi bore feed pressure at 21° C. The fibers exhibitedthe following gas separation properties while producing an inertsenriched product stream containing 95% inerts: O₂ Permeance=66 GPU;O₂/N₂ Selectivity=2.7.

The fibers were treated to seal defects in the dense separating layerand retested as described in the Comparative Example 1. The fibersexhibited the following gas separation properties while producing aninerts enriched product stream containing 95% inerts: O₂ Permeance=11GPU; O₂/N₂ Selectivity=6.2.

The above permeation data indicate that the composite fibers spun from asingle phase core solution could be post-treated up to a selectivitywhich is closer to the full O₂/N₂ selectivity of the MATRIMID separatingpolymer. This is contrary to the post-treated permeation data obtainedwith composite fibers spun from a two-phase core solution as discussedin the Comparative Example 1.

EXAMPLE 2

The failure pressure of the composite fibers described in ComparativeExample 1 and Example 1 was measured by pressurization from the tubeside. The fibers were subjected to increasing levels of N₂ pressure fromthe bore side and the N₂ permeate flow rate was measured at eachpressure. These measurements were made at room temperature. These dataare summarized in FIG. 2.

The data in FIG. 2 clearly indicate that the failure pressure of thecomposite fibers spun with a single-phase core solution is greater than500 psi, whereas the failure pressure of the fibers spun with atwo-phase core solution is about 200 psi. In these measurements, thecomposite fiber failure pressure corresponds to the bore N₂ pressure atwhich point the N₂ pressure versus the N₂ permeate flow rate relationbegins to deviate from linear functionality and starts to increaseabruptly. The failure pressure of the composite fibers spun from thesingle-phase core solution is more than twice that of the counterpartsspun from the two-phase core solution. This is a very critical issuebecause the ability of hollow fiber membranes to withstand highpressures for prolonged times is a key factor in commercialapplications.

The mechanical properties of the same fiber samples were also measuredin extension at room temperature. The fiber mechanical properties aresummarized in Table 5 below.

TABLE 5 FIBER MECHANICAL PROPERTIES Elastic Yield Maximum Maximum CoreSolution Modulus Stress Stress Strain Phase Behavior (KSI) (KSI) (KSI)(%) Two-Phase 134 3.1 4.7 57 Single-Phase 90 3.8 5.8 144

In Table 5, the elastic modulus was measured in extension according toASTM D2256 at room temperature. Yield stress was measured in extensionat room temperature, which is defined as the point of intersection ofthe tangent of the initial high slope portion of the stress strain curveand the tangent of the immediately following nearly flat portion of thestress strain curve. The measurements were performed at a strain rate of25% per minute. The two-phase core solution is as described inComparative Example 1, and the single-phase core solution is asdescribed in Example 1.

Although the composite fibers spun from the two-phase core solutionexhibit a higher elastic modulus, the composite fibers spun from thesingle-phase core solution exhibit higher yield stress, maximum stressat break, and significantly higher maximum strain at break. Since it isreasonable to assume that creep starts when the fiber is stressed beyondits yield stress and therefore that the relevant material property indetermining the fiber failure pressure is the yield stress, one wouldexpect that the fibers spun from the two-phase core solution wouldexhibit inferior failure pressure as compared to their counterparts spunfrom the single-phase core solution. This is consistent with the failurepressure data shown in FIG. 2.

Furthermore, the short elongation at break which the fibers spun fromthe two-phase core solution exhibit would severely limit the mechanicalhandling of the fibers after the spinning operation such as bobbinwinding, skeining, and fabrication of hollow fiber permeators.

COMPARATIVE EXAMPLE 2

Composite fibers were spun by using the same core and sheathformulations, and at the same spinning process conditions described inthe Comparative Example 1, except the air-gap length through which thenascent fiber traveled was kept at 2 cm. The fibers were washed,dehydrated, and tested for mixed gas O₂/N₂ (21/79) with 100 psi borefeed pressure at 21° C.

The fibers exhibited the following gas separation properties whileproducing an inerts-enriched product stream containing 95% inerts: C₂Permeance=79 GPU; O₂/N₂ Selectivity=1.5.

The fibers were treated to seal defects in the dense separating layerand retested as described in the Comparative Example 1. The fibersexhibited the following gas separation properties while producing aninerts-enriched product stream containing 95% inerts: O₂ Permeance=16GPU; O₂/N₂ Selectivity=2.5.

The above permeation data also depicts that the composite fibers spunfrom a two-phase core solution could not be post-treated to the fullO₂/N₂ selectivity of the MATRIMID separating polymer.

EXAMPLE 3

Composite fibers were spun by using the same core and sheathformulations, and at the same spinning process conditions described inthe Example 1, except the air-gap length through which the nascent fibertraveled was kept at 2 cm. The fibers were washed, dehydrated, andtested for mixed gas O₂/N₂ (21/79) with 100 psi bore feed pressure at21° C. The fibers exhibited the following gas separation propertieswhile producing an inerts-enriched product stream containing 95% inerts:O₂ Permeance=102 GPU; O₂/N₂ Selectivity=1.6.

The fibers were treated to seal defects in the dense separating layerand retested as described in the Comparative Example 1. The fibersexhibited the following gas separation properties while producing aninerts-enriched product stream containing 95% inerts: O₂ Permeance=14GPU; O₂/N₂ Selectivity=6.8.

The above permeation data also depict that the composite fibers spunfrom a single-phase core solution could be post-treated up to aselectivity which is closer to the full O₂/N₂ selectivity of theMATRIMID separating polymer.

EXAMPLE 4

The fibers described in the Comparative Example 2 and Example 3 weretested for O₂/N₂ permeation properties with 100 psi tube feed pressureafter exposure of the fibers to 300 and 500 psi for 30 minutes intervalsfor determination of their respective failure pressures. For this typeof measurement, the failure pressure of the fiber is defined as themaximum bore pressure the fiber can withstand with full retention of theinitial selectivity. The permeation data for these fibers are summarizedin Table 6 below.

TABLE 6 TUBE FEED O₂/N₂ PERMEATION PROPERTIES AS A FUNCTION OF PRESSURE¹O₂ Performance O₂/N₂ Fiber State (GPU) Selectivity Remarks Two-Phase² 162.5 Initial Two-Phase² 29 1.5 After 30 minutes of exposure to 300 psi.Two-Phase² 161 1.1 After 30 minutes of exposure to 500 psi.Single-Phase³ 14 6.8 Initial Single-Phase³ 15 6.8 After 30 minutes ofexposure to 300 psi. Single-Phase³ 16 6.6 After 30 minutes of exposureto 500 psi. ¹All permeation data were measured at 100 psig feed pressureat 21° C. while producing 90 to 96% inerts-enriched air. ²Two-phasesolutions are those described in Comparative Example 2. ³Single-phasesolutions are those described in Example 3.

It is clear from the data summarized in Table 6 that the compositefibers spun with the two-phase core formulation failed at a borepressure less than or equal to 300 psi, whereas the fibers spun with thesingle-phase core solution essentially retained full selectivity afterexposure to 500 psi.

This example also illustrates the superior mechanical properties of thepolyimide blend composite hollow fibers spun from a single-phase coresolution.

This invention has been described in detail with particular reference toexamples and preferred embodiments thereof, but it is to be understoodthat variations and modifications may be resorted to as will be apparentto those skilled in the art. Such variations and modifications are to beconsidered within the purview and scope of the claims appended hereto.

What is claimed is:
 1. A membrane which is made from a polymer blendsolution comprising a plurality of polyimides, a solvent, and a criticalsolution temperature (CST) adjustment agent selected from the groupconsisting of alkali or alkaline earth metal halides, triethylamine, andcombinations thereof, said CST adjustment agent being present in anamount effective to enhance the miscibility of said plurality ofpolyimides in solution.
 2. The membrane according to claim 1, which isin the form of hollow fibers.
 3. The membrane according to claim 1,wherein the polymer blend solution which comprises a first Polymer Ahaving the following repeating unit:

and a second Polymer B having the following repeating unit:


4. The membrane according to claim 3, which comprises between 80% and95% by weight of Polymer A based on the total polymer content in thesolution.
 5. The membrane according to claim 1, wherein the CSTadjustment agent is an alkali or alkaline earth metal halide.
 6. Themembrane according to claim 1, wherein the CST adjustment agent isZnCl₂, CaBr₂, LiCl, triethylamine, or combinations thereof.
 7. Themembrane according to claim 1, wherein the polymer blend solution has aHCST that is at least 10° C. lower than the same blend solution withouta CST adjustment agent.
 8. The membrane according to claim 1, whereinthe solvent comprises N-methylpyrrolidone.
 9. A method of making themembrane according to claim 1, which comprises coextruding said polymerblend solution with a sheath solution to form composite hollow fibers.10. A membrane which is made from a polymer blend solution comprising aplurality of polyimides and a solvent, wherein said solution has beenannealed at a temperature effective to lower a higher critical solutiontemperature (HCST) of the solution.
 11. The membrane according to claim1, wherein the temperature is between 50° C and 140° C.
 12. The membraneaccording to claim 11, wherein the temperature is between 70° C and 100°C.
 13. The membrane according to claim 10, wherein the solutioncomprises a critical solution temperature (CST) adjustment agent. 14.The membrane according to claim 13, wherein the CST adjustment agent isZnCl₂, CaBr₂, LiCl, triethylamine, or combinations thereof.
 15. Themembrane according to claim 10, wherein the solvent comprisesN-methylpyrrolidone.
 16. The membrane according to claim 10, which is inthe form of hollow fibers.
 17. A method of making the membrane accordingto claim 10, which comprises coextruding said polymer blend solutionwith a sheath solution to form composite hollow fibers.